1. Field
The present invention relates generally to position determination, and more specifically to a method and apparatus for creating and using a base station almanac (BSA) for position determination.
2. Background
It is often desirable, and sometimes necessary, to know the position of a wireless user. For example, the Federal Communications Commission (FCC) has adopted a report and order for an enhanced 911 (E-911) wireless service that requires the position of a wireless terminal (e.g., a cellular phone) to be provided to a Public Safety Answering Point (PSAP) each time a 911 call is made from the terminal. In addition to the FCC mandate, service providers may use location services (i.e., services that identify the position of wireless terminals) in various applications to provide value-added features.
The position of a wireless terminal may be determined based on various systems. One such system is the well-known Global Positioning System (GPS), which is a “constellation” of 24 well-spaced satellites that orbit the earth. Another such system is a cellular communication system that includes a number of base stations to support communication for a number of user terminals. A position estimate (also commonly referred to as a “fix”) for the terminal may be obtained based on (1) distances or ranges from the terminal to a sufficient number of transmitters (typically three or four) and (2) the locations of these transmitters. Each transmitter may correspond to a GPS satellite or an earth-bound base station in the cellular system. The range to each transmitter may be estimated based on a signal received by the terminal from the transmitter. The location of each transmitter can typically be ascertained based on the identity of the transmitter, which may also be obtained from the signal received from the transmitter.
An almanac is typically maintained for each system used for position determination. The almanac contains various types of information that are pertinent for position determination such as the identity of each transmitter in the system, the location of the transmitter, timing information for the transmitter, and so on. The almanac information may be used in various manners for position determination. For example, the locations of the transmitters in the almanac may also be used in combination with measurements (e.g., ranges) for the transmitters to derive a final position estimate for the terminal.
A wireless network operator may maintain a base station almanac for base stations and repeaters in a cellular network. The base station almanac for a Code Division Multiple Access (CDMA) network that implements IS-95 or IS-2000 standard is specified in a document TIA/EIA/IS-801-1, entitled “Position Determination Service Standards for Dual Mode Spread Spectrum Systems, Addendum 1.” The base station almanac defined by IS-801 has a number of shortcomings that limit its use and compromise the quality of the position estimates obtained with the base station almanac. For example, the identifiers for the base stations are defined with parameters that are specific to IS-95 and ANSI-41 network interface. The IS-801 base station almanac thus cannot generally support other air interface protocols besides IS-801. Other limitations of the IS-801 base station almanac are described below.
There is therefore a need in the art for an improved base station almanac for position determination.